Apparatus and method of operating a magnetohydrodynamic generator



FIPESGZ GR 933199289].

May 9, 1967 BURHQRN ET AL 3,319,091

APPARATUS AND METHOD OF, OPERATING A MAGNETOHYDRODYNAMIC GENERATOR Filed Oct. 9, 1963 1mm ---I I FIG. 2

f ,i/ FIG. 3 1o X FIG. FIG. 5

United States Patent 8 Claims. e1. 310-11 Our invention relates to magnetohydrodynamic generators and more particularly to apparatus and method of operating such generators.

A magnetohydrodynamic generator (MHD generator) usually comprises a channel through which hot electrically conducting gas (plasma), at a temperature of about 3000 C. is passed at a high speed. The channel is intersected by a magnetic field so that an electric field is induced perpendicular to the magnetic field and perpendicular to the flow direction of the gas. In order to draw off electric energy, electrodes must be arranged in the conducting zone of the gas jet; the maximum possible current intensity is proportional to the surface area of the electrodes. Since the conductivity in plasma primarily depends on the electron motion (ion conductivity being less by about a factor of 1000), a thermal emission current at the outer positive electrode must ensure the current passage in the plasma.

From the standpoint of primary energy there are two different types of MHD generators that are to be distinguished. In one type, plasma is produced by combustion of oil, coal dust or similar fuels with oxygen or air. In the other type of generator, the gas is heated in a heat exchanger in which the heat can for example be provided by a nuclear reactor.

A decided limit to the life expectancy of such generators is caused by the fact that the conventional electrodes are not durable at the high temperatures of the plasma. In this regard, one must consider that the plasma partly consists of oxidizing atmospheres so that the electrodes that are employed, such as graphite for example, burn up in a relatively short time.

In an effort to avoid these difficulties, intensive cooling of the electrodes has been provided. For example, in the periodical Nature of Feb. 3, 1962, pages 467 and 468, there is reported a proposal to employ water-cooled copper electrodes with a surface temperature of about 400 C.

It has now become evident that the performance of an MHD generator employing such forced cooling as compared to the performance of an MHD generator with for example uncooled graphite electrodes declines by about one-tenth. This decline in performance is caused as follows:

The plasma stream contains electrons and positive ions of seed material, for example potassium or potassium oxide ions. The conductivity of the plasma will be high due to the mobility of the electrons, which is at higher orders of magnitude than that of the heavy positive ions, if the current transfer from the positive electrode to the negative electrode within the plasma can be taken over by the electrons. It is consequently necessary that the positive electrode be able to emit the necessary amount of electrons. When the electrodes are greatly cooled, a condensation of substances containing potassium oxide and potassium carbonates is formed thereon. Since the condensation layer that is formed is a poor heat conductor, this layer becomes so thick that its outer surface takes on a temperature at which equilibrium between condensation and vaporization prevails. This temperature 3,319,091 Patented May 9, 1967 is higher than 1200 C. Since potassium and potassium oxide have a low electron outlet potential this temperature would sufiice for assuring a sufficiently high emission density (at least 1 A./cm. This layer, however, has a poor electrical conductivity and its resistance increases with thickness and with decrease in temperature. The layer, therefore, performs like a poorly conducting semiconductor, in which a greater part of the voltage produced in the MHD generator breaks down.

It is accordingly an object of our invention to provide an apparatus and method of operating an MHD generator which takes into consideration the fact that the condensation layer surface temperature level has a decisive influence on the thickness of the deposited layer and therewith on the generator efficiency, and it is consequently our object to increase the efiiciency of such generators by means of our invention.

To this end and according to a feature of our invention we provide apparatus and method for adjusting the cooling of the electrodes to a value at which the electrode is at a temperature just beneath the melting point of the electrode material and the marginal zones of plasma bordering the electrodes are maintained at a temperature which permits the formation out of the constituents of the plasma at the electrode surface of a thin layer capable of emission, whose thickness remains in a constant equilibrium state due to vaporization and new precipitation.

According to another feature of our invention we provide apparatus and method by means of which the electrodes in an MHD generator are not force-cooled but are given a measured cooling.

With usual coolants, such as water for example, the quantity of coolant and the coolant temperature as well as the density and therewith the surface temperature of the electrode metal, are chosen so that on the one hand no appreciable corrosion or burn-off occurs and on the other hand, a thin condensation layer is formed which provides satisfactory electron emission for a relatively small resistance of the layer as compared to the resistance of the plasma. The temperature of the metallic electrode surface that is necessary therefor is at least 1100 C.

The so-called noble steels (Thermax steel) that are ignition-resistant and relatively nonoxidizing have been found suitable for the electrodes and have in part also been suggested for use under forced cooling conditions. In addition, hard metals also as well as other materials with especially high melting points, such as molybdenum disilicide for example, are also suitable.

At these temperatures the potassium condensate emits so many electrons onto the electrodes that current densities of 1 A./cm. (ampere per centimeter square) to 10 A./cm. according to the surface temperature, can be maintained, whereby the plasma conductivity is determined only by electron transfer. The condensation layer thereby formed is so thin that no significant voltage drop at an electrode spacing of only 2 cm. (which corresponds to a generator voltage of about 20 volts) takes place any more.

On the other hand, the electrode surface temperature must not be so high that no emitting layer can be formed. This limiting temperature is higher than about L350 C. In practice, however, a temperature exceeding 1800 to 1350 C. produces no significant results, since heat losses due to the measured cooling of the surfaces are of little importance.

The electric power density that is removable per unit of volume of the reaction space when measured cooling is employed in accordance with the invention, is practically as great as when using electrodes that are not cooled, for example uncooled graphite electrodes. With a channel cross section of 2X2 cm. under conditions, therefore, in which the proportion of surface to volume is very unfavorable, average power densities of approximately 10 watt/cm. can be achieved at a plasma temperature of 2700 C. at the inlet and 2400 C. at the outlet of the channel, a flow speed of about 1000 m./sec. and a magnetic field strength of about 14 koe. (kilo-oersted). These values are close to one order of magnitude above the values achieved with force-cooled electrodes (1 to 1.4 watt/cm?) published to date.

Other features which are considered as characteristic for the invention are set forth in the appended claims. The invention, however, together with additional objects and advantages thereof, will be best understood from the following description when read in connection with the accompanying drawings, in which:

FIG. 1 is a schematic perspective drawing of two electrodes employed in the apparatus and in carrying out the method of this invention;

FIG. 2 is a schematic drawing of an electrode on which is superimposed a graph showing the temperature distribution along the electrode;

:FIGS. 3 and 4 are cross-sectional views of modified electrodes used in the apparatus and in carrying out the method of this invention; and

FIG. 5 is a schematic layout of a system for making use of heat losses in accordance with the method of this invention.

Referring to the drawings and first particularly to FIG. 1, there is shown schematically two electrodes 1, 2 which define a channel 3 between each other and between a pair of walls that are arranged above and beneath the electrodes but are not represented in the drawing. The electrodes are provided with cooling channels 4 and 5 through which a coolant, such as water for example, is conducted. In an experimental assembly that was tested, the width of the electrode material was mm., the height (the same as the channel height) mm., and the length of the electrodes 200 mm. The width of the cooling channel was 7 mm. During operation, 250 watts per cm. of electrode surface were removed by cooling.

The temperature distribution along the electrodes is shown schematically in FIG. 2. The temperature increases from the cooling water temperature of about 40 C. up to a value barely below the melting point of the electrode material at the metallic surface of the electrodes. For example, Thermax 8 A. has a melting point of about 1400 C. When a vapor (steam) is employed as coolant, the thickness of the electrode material must be reduced accordingly, whereby the heat transfer out of the plasma remains almost constant with constant surface temperature.

In FIG. 2 there is also shown a layer or coating 6 which is deposited on the surface of the electrodes during operation, and in which a large temperature gradient prevails. The surface temperature of the layer 6 is therefore considerably above the surface temperature of the electrode material. A relatively high plasma temperature is accordingly produced at the surface of the layer 6 so that the conductivity of the plasma remains high. The temperature in the plasma increases to approximately 3000 C. toward the middle (M in FIG. 2) of the channel.

The relatively high surface temperature of the steel electrode causes the formation of only a very thin coating so that the high specific resistance thereof does not have a damaging effect upon the performance or efficiency of the generator. The thickness of the layer 6 is maintained constant due to the development of a state of equilibrium between vaporization of the previously deposited layer and new deposits of the various constituents of the inner atmosphere of the generator. These constituents also emit sufiicient electrons at the relatively high temperature, a characteristic which is of particular importance for the electrode which serves as cathode.

As FIG. 3 illustrates, it is possible to provide channels 7 in the electrode through which one can send air or oxygen, for example, for preheating the same. The cooling channel 5 can also be eliminated in accordance with the embodiment shown in FIG. 4, by providing channels 8 instead in the electrode material into which cooling water under high pressure (for example 200 atmospheres) is injected. The steam vapor arising therefrom can also serve a second purpose, that of producing energy, for example by means of a steam turbine. It is therefore advantageous to select a larger number of channels of small diameter which are disposed parallel to or one behind the other.

Utilization of the heat removed by cooling is of economic importance since about 8% of the energy that is not added to the MHD channel is withdrawn by the cool- FIG. 5 shows a schematic drawing of an assembly for exploiting this lost heat. The generator 9 with the burner chamber 10 is loaded with a load resistance '11 which is connected to both electrodes over a measuring instrument 12. The oxidant, for example air, is driven by a pump 13 through the channels in the electrode bodies and is thereafter fed to the burner chamber 10. The coolant cycle is provided with a pump .14 and a heat exchanger 15. The heat energy that is removed is employed for heating the fuel, for example oil, in the heat exchanger, the fuel being forced out of the supply container 16 by means of a pump 17 into the burner chamber .10.

The cooling capacity is optimized by adjustment of the pumping output, through reducing valves or the like, in dependence upon the electrical energy produced by the MHD generator as indicated by the reading on the measuring instrument 12.

Of course the heat energy removed by cooling can be used to heat the oxidant instead of the fuel prior to its entry into the burner chamber 10, or can be used to heat both the oxidant and the fuel.

In order to increase the effectiveness of the steel electrodes 1, 2 their surface can be platinized to resist the high temperatures and can be roughened to provide an increase in the electric energy that can be drawn off.

While the invention has been illustrated and described as a method of operating a magnetohydrodynamic generator, it is not intended to be limited to the details .shown, since various modifications and structural changes may be made without departing from the spirit of the present invention and within the scope and range of equivalence of the following claims.

We claim:

1. For operating a magnetohydrodynamic generator wherein a plasma current flows between a pair of electrodes, a method which comprises cooling the electrodes to a temperature level slightly below the melting point of the material constituting the electrodes and maintaining the electrodes at said temperature during the operation of the generator, marginal zones of the plasma bordering the electrodes being simultaneously maintained at a temperature at which a thin emitting layer, having a resistivity that is small compared to that of the plasma, is formed from the plasma constituents on the electrode surfaces, vaporization of said layer and further deposition of said constituents on the electrodes being in a state of equilibrium whereby the thickness of said layer remains constant.

2. For operating a magnetohydrodynamic generator wherein a plasma current flows between a pair of electrodes, a method which comprises cooling the electrodes to a temperature :level slightly below the melting point of the material constituting the electrodes, the cooling supplied being the most favorable for the electric energy generated by the generator within the thermal stability range of said electrode material, and maintaining the electrodes at said temperature during the operation of the generator, marginal zones of the plasma bordering the electrodes being simultaneously maintained at a temperature at which a thin emitting layer, having a resistivity that is small compared to that of the plasma, is formed from the plasma constituents on the electrode surfaces, vaporization of said layer and further deposition of said constituents on the electrodes being in a state of equilibrium whereby the thickness of said layer remains con stant.

3. For operating a magnetohydrodynamic generator wherein a plasma current from an oxidized fuel source flows between a pair of electrodes, a method which comprises cooling the electrodes to a temperature level slightly below the melting point of the material constituting the electrodes, preheating at least one of the fuel and its oxidizer with the heat removed by said cooling, and maintaining the electrodes at said temperature during the operation of the generator, marginal zones of the plasma bordering the electrodes being simultaneously maintained at a temperature at which a thin emitting layer, having a resistivity that is small compared to that of the plasma, is formed from the plasma constituents on the electrode surfaces, vaporization of said layer and further deposition of said constituents on the electrodes being in a state of equilibrium whereby the thickness of said layer remains constant.

4. For operating a magnetohydrodynamic generator wherein a plasma current flows between a pair of electrodes, a method which comprises cooling the electrodes to a temperature level slightly below the melting point of the material constituting the electrodes, converting Water to steam for driving a turbine with the heat removed -by said cooling, and maintaining the electrodes at said temperature during the operation of the generator, marginal zones of the plasma bordering the electrodes being simultaneously maintained at a temperature at which a thin emitting layer, having a resistivity that is small compared to that of the plasma, is formed from the plasma constituents on the electrode surfaces, vaporization of said layer and further deposition of said constituents on the electrodes being in a state of equilibrium whereby the thickness of said layer remains constant.

5. In a magnetohydrodynamic generator, a pair of electrodes consisting of highly heat-resistant steel having a melting point of about 1400 C., and means for cooling said electrodes to a temperature level slightly below said melting point and maintaining said electrodes at said temperature level during the operation of the generator, said cooling means being adapted to simultaneously maintain the marginal zones of the plasma bordering said electrodes at a temperature at which a thin emitting layer, having a resistivity that is small compared to that of the plasma, is formed from the plasma constituents on the electrode surfaces, and at which vaporization of said layer and further deposition of said constituents on said electrodes is in a state of equilibrium whereby the thickness of said layer,

References Cited by the Examiner UNITED STATES PATENTS 2,459,579 1/1949 Noel 3 l3213 2,904,717 9/ 1959 Kerstetter 313- 355 2,965,793 12/1960 Feaster 3 l3346 OTHER REFERENCES Nature, Feb. 3, 1962, pp. 467 and 468.

Engineering Aspects of Magnetohydrodynamics: Edited by Manual and Matther from Proceedings of Second Symposium, March 9 and 10, 1961, pp. 149 and 150.

MILTON O. HIRSHFIELD, Primary Examiner.

D. X. SLINEY, Examiner. 

1. FOR OPERATING A MAGNETOHYDRODYNAMIC GENERATOR WHEREIN A PLASMA CURRENT FLOWS BETWEEN A PAIR OF ELECTRODES, A METHOD WHICH COMPRISES COOLING THE ELECTRODES TO A TEMPERATURE LEVEL SLIGHTLY BELOW THE MELTING POINT OF THE MATERIAL CONSTITUTING THE ELECTRODES AND MAINTAINING THE ELECTRODES AT SAID TEMPERATURE DURING THE OPERATION OF THE GENERATOR, MARGINAL ZONES OF THE PLASMA BORDERING THE ELECTRODES BEING SIMULTANEOUSLY MAINTAINED AT A TEMPERATURE AT WHICH A THIN EMITTING LAYER, HAVING A RESISTIVITY THAT IS SMALL COMPARED TO THAT OF THE PLASMA, IS FORMED FROM THE PLASMA CONSTITUENTS ON THE ELECTRODE SURFACES, VAPORIZATION OF SAID LAYER AND FURTHER DEPOSITION OF SAID CONSTITUENTS ON THE ELECTRODES BEING IN A STATE OF EQUILIBRIUM WHEREBY THE THICKNESS OF SAID LAYER REMAINS CONSTANT. 